Study designed to compare the efficacy of two regimens of dapsone and atovaquone in preventing or delaying the onset of histologically proven or probable PCP in trimethoprim/sulfonamides-intolerant, HIV infected patients with CD4+ cell counts of <200mm3 or < 15 percent of the total lymphocyte count, and in preventing or delaying the recurrence of PCP in individuals who have experienced previous episodes, the toxicities of the two regimens, the incidence of the combined endpoint of PCP, extrapulmonary pneumocystosis and death in the two treatment arms, the incidence of toxoplasmosis in the two treatment arms and the mortality rates in the two treatment arms.